This invention relates to improvements in electromagnetic clutches and is particularly, but not exclusively, suitable for vehicles intended to provide an additional or auxiliary drive output to an auxiliary piece of equipment.
Conventional agricultural tractors, for example, are normally equipped with a rear power take-off, provided by a shaft which can be drivenly connected to the engine, and which can be used for operating numerous auxiliary devices and equipment, such as revolving harrows, cutters, mowing machines, drills, pumps and the like, towed by the tractor.
The use of such auxiliary devices, nevertheless, is frequently associated with disadvantages due, for example, to irregularity of the ground, which can contain stones, roots and the like and against which the devices can jam. For preventing such obstacles from causing breakages of the shaft of the power take-off, which is frequently of limited cross-section, clutches are usually employed for coupling such a shaft to the engine, which clutches slip at a load less than that which is dangerous for said shaft.
If this slip is then not signalled to the driver, overheating of the clutch occurs and this can result in irreparable damage.
To protect the clutch, therefore, it is necessary to use a coupling means which, in the case where the maximum transmissible torque is exceeded or in the case where the clutch slips, automatically causes disengagement of the clutch.